1. Field of the Invention
The present invention relates generally to a perpendicular magnetic recording technique and, more particularly, to a perpendicular magnetic recording medium having recording layers formed on two surfaces of a substrate.
2. Discussion of Background
Recently, a perpendicular magnetic recording technique has been studied as a new technique which replaces a conventional magnetic recording technique. According to this technique, residual magnetization is formed in a direction substantially perpendicular to a film surface of a magnetic recording layer of a recording medium, thus realizing high-density magnetic storage of data.
A disk-shaped magnetic recording medium which can record/reproduce data on/from two surfaces thereof is preferably used as a recording medium in the perpendicular magnetic recording technique. This disk-shaped magnetic recording medium is known to persons skilled in the art as a "double-sided recording type disk" or a "double-sided diskette". According to the recording disk of this type, first and second magnetic recording films are formed on two surfaces, respectively, of a disk-shaped substrate formed of, e.g., a flexible base layer of a plastic material or the like. These magnetic recording films have an axis of easy magnetization in a direction perpendicular to their surfaces and are, e.g., cobalt-chromium (Co-Cr) films. When a changing magnetic field corresponding to recording data is applied to either of the Co-Cr films by a magnetic head, residual magnetization is formed in correspondence with the magnetic field strength so as to be perpendicular to the film surface, thereby recording data.
When a perpendicular magnetic recording disk as described above is manufactured, Co-Cr magnetic films are generally formed on a substrate or base layer by sputtering or deposition. Since the magnetic films must be formed on two surfaces of the base layer, it is difficult to match physical characteristics of the two magnetic films in this manufacturing process. If waving of the film, i.e., a so-called "curl" phenomenon (this phenomenon is considered to easily occur when a double-sided disk is manufactured) occurs, an intimate contact movement of the magnetic head to the surface of the magnetic film (i.e., uniform head touch) cannot be obtained. The recording head touch onto the surface of the magnetic film is very important for the perpendicular magnetic recording technique with high recording density. The "curl phenomenon" prevents a coincidence between the characteristics of two magnetic films or recording films (i.e., noncoincidence or nonuniformity of the recording current or reproduced output between two magnetic films undesirably occurs). This renders the manufacture of a perpendicular magnetic recording medium more difficult.